(1) Field of the Invention
The present invention relates to a novel adhesive suitable for jointing a variety of articles, especially shaped articles of cured polyolefin and a method of jointing articles of cured polyolefin using the adhesive.
(2) Description of the Prior Art
Since cured polyolefin is superior to uncured polyolefin in heat resistance, mechanical strength, especially in resistance to environmental stress cracking, shaped articles of cured polyolefin are increasing more and more in kind and amount product. For instance, a cured polyolefin pipe appear useful as gas pipe, tap water pipe, and hot water pipe.
There have been proposed several methods of jointing cured polyolefin pipe without using metallic joints which have been used heretofore and have shortcomings, such as high cost and corrosiveness. Those methods, however, have not found practical use yet. For example, a method of heat adhering two cured polyolefin pipes has the drawback that because the cured polyolefin does not melt well due to its crosslinked molecular structure, joining strength is not very good so that flow of a pressurized liquid or heat cycles of the pipe often cause disconnection of the joint portion.
A method of using an adhesive consisting of a polar organic material has been tested. However, its adhesion is not enough, owing to the non-polarity or low polarity of cured polyolefin.
On the other hand, experiments of the inventors of the present invention had revealed that ordinary polyolefin, such as polyethylene, will function as an adhesive for cured polyolefin. Ordinary polyolefin, which has excellent adhesion for a short period of time, such as for about one month after the joining, however, has a problem that its adhesion gradually deteriorates. In this case, at an early stage after joining, temporal entanglements and physico-chemical adherence between the molecules on the surface of a cured polyolefin article and the polyolefins of adhesive provide excellent joining strength, which is gradually weakened by the action of various forces such as a force due to thermal expansion and contraction of the joint portion.